In general, a non-aqueous electrolyte power storage element includes an electrode assembly having a positive electrode and a negative electrode, a case containing the electrode assembly, and an electrolyte solution contained in the case. The positive electrode includes a metal foil, and a positive active material that covers a surface of the metal foil. Examples of the positive active material include various types. One example that is used is an active material in which two phases, i.e. a first phase that exists before discharge and a second phase that appears later, coexist (two-phase reaction type active material). Examples of the two-phase reaction type active material include LiFePO4 (lithium iron phosphate) (cf., patent document 1).
Charging and discharging of a non-aqueous electrolyte power storage element is performed by lithium ions moving between a positive electrode and a negative electrode. With the two-phase reaction type active material, lithium ions are not easily diffused, upon charging and discharging, the lithium ions moved to the positive electrode are diffused unevenly within the two-phase reaction type active material of the positive electrode. As a result of this, in the positive electrode, a region in which the lithium ions are diffused and thus contribution to charging and discharging is made, and a region in which the lithium ions are not diffused and thus no contribution to charging and discharging is made are formed. Then, it become difficult for the lithium ions to be diffused in the positive electrode after repeated charging and discharging to the non-aqueous electrolyte power storage element, the region in which no contribution to charging and discharging is made gradually increases.
When the region in which no contribution to charging and discharging is made increases in the positive electrode in this manner, in the non-aqueous electrolyte power storage element, degradation of a charge-discharge performance, which is greater than degradation of a charge-discharge performance produced by deterioration of the positive active material, an electrolyte solution, and the like due to repetition of charging and discharging, is produced. If charging and discharging are continued repeatedly in this state, the region in which no contribution to charging and discharging is made increase even more, and deterioration of the positive electrode becomes apparent.